Proton beam radiotherapy employs a cyclotron to energize protons and uses magnetic fields to direct the protons to the tumor. Proton therapy is most precise form of radiation treatment available for cancer. Medical researchers are excited about this form of treatment because it is noninvasive and painless and it promises to leave surrounding health tissue and organs intact and unharmed.
Radiation therapy (also called radiotherapy, X-ray therapy, or irradiation) is the use of ionizing radiation to kill cancer cells and shrink tumors. It can be delivered internally (brachytherapy) or externally (external beam radiotherapy). Radiation therapy injures or destroys cells in the area being treated (the “target tissue”) by damaging their genetic material, making it impossible for these cells to continue to grow and divide. Although radiation damages both cancer cells and normal cells, most normal cells can recover from the effects of radiation and function properly. The goal of radiation therapy is to damage as many cancer cells as possible, while limiting harm to nearby healthy tissue. Hence, it is given in many fractions, allowing healthy tissue to recover between fractions.
Radiation therapy may be used to treat almost every type of solid tumor, including cancers of the brain, breast, cervix, larynx, lung, pancreas, prostate, skin, stomach, uterus, or soft tissue sarcomas as well has leukemia and lymphoma. The radiation dose given to each site depends on a number of factors, including the radiosensitivity of each cancer type and whether there are tissues and organs nearby that may be damaged by radiation. As with every form of treatment, radiation therapy is not without side effects. Proton therapy is another form of external-beam radiation treatment.
Mechanism of proton therapy
Proton therapy, like all forms of radiotherapy, works by aiming energetic ionizing particles at the target tumor. These particles damage the DNA of cells and ultimately cause their death. Because of their high division rates and their reduced ability to repair damaged DNA, cancerous cells are particularly vulnerable to this attack on their DNA. As protons do not scatter easily in the tissue there is very little lateral dispersion; the beam stays focused on the tumor shape without much lateral damage to surrounding tissue.
Differences from conventional X-ray therapy
Both standard x-ray therapy and proton beams work on the principle of selective cell destruction. The major advantage of proton treatment over conventional radiation, however, is that the energy distribution of protons can be directed and deposited in tissue volumes designated by the physicians-in a three-dimensional pattern from each beam used. This capability provides greater control and precision and, therefore, superior management of treatment. Radiation therapy requires that conventional x-rays be delivered into the body in total doses sufficient to assure that enough ionization events occur to damage all the cancer cells. The conventional x-rays lack of charge and mass, however, results in most of their energy from a single conventional x-ray beam being deposited in normal tissues near the body’s surface, as well as undesirable energy deposition beyond the cancer site. This undesirable pattern of energy placement can result in unnecessary damage to healthy tissues, often preventing physicians from using sufficient radiation to control the cancer.
How is proton therapy given?
The three-dimensional information is usually obtained by performing a computed tomography (CT) scan through the region of interest (chest, pelvis, etc.) with images (sometimes called slices) being taken at 2- to 3-millimeter intervals. Before performing the CT scan, some type of immobilization device is made for the patient, so as to reproduce the patient’s treatment position each day.
Typical immobilization devices include full-body moulds (form-fitting foam liners surrounded by rigid plastic shells), for patients with tumors below the neck, and custom-manufactured masks, for patients with eye, brain, and head tumor abnormalities.
Tumors treated with proton therapy
The earliest treatment success by proton therapy was in the treatment of choriodal malignant melanomas of the eye, where earlier enucleation (removal of the eye) was the only treatment available. Other tumors successfully handled are that of the brain, head, neck, lung and prostrate where therapy is often combined with other cancer treatment modalities. Specifically some of the brain and cranial base tumors treated are acoustic neuromas, ependymomas, gliomas and meningiomas, among spinal tumors those such as chordomas and chondrosarcomas and gastrointestinal cancers like unresectable liver tumors have also been effectively managed through proton therapy. Some of the pediatric tumors that respond well to proton therapy are astrocytomas, ependymomas, germinomas, medulloblastomas and some sarcomas.
Proton beam therapy can be used in conjunction with x-ray therapy. Such combinations are typically done to areas with large tumors. The combination therapy often results in high remission rates. Because the primary tumor may put out its colonies into nearby lymph nodes, doctors sometimes feel combining proton therapy with that of conventional radiotherapy like x-ray is advisable.
Advantages of proton therapy
Proton therapy is of interest because of its ability to accurately target and kill tumors, both near the surface and deep seated within the body, while minimizing damage to the surrounding tissues. For this reason, it is favored for treating certain kinds of tumors where conventional X-ray radiotherapy would damage surrounding radio-sensitive tissues to an unacceptable level (optical nerve, spinal cord, central nervous system, head, neck and prostate). This is of significance in the case of pediatric patients where long term side effects such as residual occurrence of secondary tumors resulting from the overall radiation dose to the body are of great concern.